In order to become an initiate of the sect of the Sleeper you need to impress the Baals. They wear distinctive clothing, and require you to perform sometimes complicated tasks. You cannot talk directly to them, so you must find out how to impress them by talking to other people.
Talk to Lester and ask all the topics he offers. When you've run out of topics ask him to lead you to the temple. As soon as you're a short distance away from Namib ask him how to earn Namib's favour, and he will tell you a lie to rehearse. Now tell him you've changed your mind, and he'll go back to the entrance.
Go back to the entrance and he'll speak to you. Say you've renounced the old gods, and everything else will play out from there. Go and stand in front of Namib, who will say you must indeed be worthy. Tell him you want to join the Brotherhood.
He will give you a Prepared Weed. Go back to the Sect Camp and hand it to Baal Tyron, who will be suitably impressed by it.